We report the discovery of a planet with a high planet-to-star mass ratio in the microlensing event MOA-2009-BLG-387, which exhibited pronounced deviations over a 12-day interval, one of the longest for any planetary event. The host is an M dwarf, with a mass in the range 0.07M_{sun}_<M_host_<0.49M_{sum}_ at 90% confidence. The planet-star mass ratio q=0.0132+/-0.003 has been measured extremely well, so at the best-estimated host mass, the planet mass is m_p_=2.6 Jupiter masses for the median host mass, M=0.19M_{sun}_.